Tech Forum

First off, neither a capacitor or inductor is added to an antenna in order to change the length, they are added to match an impedance mismatch. In effect, this is LIKE changing the length IF changing the length would match the impedance of the antenna to that of the radio & feed-line. What you are really doing is counter acting the mismatch with the equal and opposite type impedance to make an impedance matching circuit, or a fixed antenna tuner, so-to-speak.

Many terms used by hams are misleading in that they really don't do what the term is saying they do. An antenna tuner doesn't tune the antenna, it transforms a complex reactance of the feed line and antenna into a resistance so that the radio can pass all its power to the antenna. The word "complex" is used because a mismatched antenna will look like a resistor and either a capacitor or an inductor. The radio wants to "see" just a simple resistor, a perfectly matched antenna to the impedance of the radio's antenna port.

All these terms may have come into being because someone somewhere along the line said, "Oh that's like changing the length of my dipole to match the impedance of my radio" and so the idea of "changing the length" stuck. I don't know how the term came about but it creates the idea that we're changing the length when we're really matching the over-all antenna and circuit resistance to that of the radio, and that's what we're after, a matched circuit so we can put all our radio energy onto the antenna and into the air.

Try reading, An Approach To Antenna Tuning by Lloyd Butler VK5BR, it explains this in more detail: http://users.tpg.com.au/users/ldbutler/Approach_Ant_Tuning.htm

An antenna becomes resonant when the energy that is racing down the wire hits the open end and is reflected back to the sending end and the transit time is equal to the time of one cycle of frequency. The open end has high voltage and low current (there can’t be any current at the open end) and the sending end has high current and low voltage. Adding inductance at the sending end will lower the resonant frequency thus making the wire appear longer. Adding capacitance to the open end will also reduce the resonant frequency. Since radiation occurs from the wire, adding these L and C elements will reduce the antenna efficiency because it is shorter than it would be if the L or C were not added.